CHAPTER XVI 



THE INFLUENCE OF THE DIVERGENCE OF THE CONTINENTS ON THE 

 DISTRIBUTION OF SPHAGNUM AND CAREX 



The predominance of Sphagnum on the middle slopes of Pico and 

 on the higher parts of San Miguel and Terceira in the Azores first 

 directed my attention to the distribution of Peat-mosses. Whilst 

 subsequently studying the matter in Warnstorf's recent monograph 

 on the Sphagnacece (Das Pflanzenreich, 1911), I became much im- 

 pressed with the fact that the Sphagnum floras of the eastern and 

 western hemispheres became more and more unlike as one receded 

 from the Arctic regions. This led me to realise that I had completely 

 ignored in my previous inquiries one of the most important factors 

 in shaping distribution, namely, that concerned with the divergence 

 of the two great land-masses of the globe from the north polar 

 area. 



From the Peat-mosses I turned to the Carices, another world- 

 ranging group of plants, and from them received the same reply. 

 It came as a surprise to me that two groups, so far removed from 

 each other in the scale of plant-life and differing so greatly in their 

 capacities for dispersal, should respond in the same way to the 

 arrangement of the two great land-masses of the Old and the New 

 Worlds. It was a relief to learn that some months had not been spent 

 in vain in the comparison of dissimilar things. From the stand- 

 point of distribution their behaviour proved to be the same, the 

 differences being only in degree. But perhaps the most welcome 

 revelation of all was that many of the difficulties associated with the 

 distribution of the higher plants, such as concern insular floras and 

 the floras of South America, Africa, and Australia, reappeared, 

 though in a less intense form, in the distribution of the humble Peat- 

 mosses. Such uniformity of behaviour, independent as it is of degree 

 of organisation and of dispersing capacity, deeply impressed me, 

 and led to my reading over again Dyer's essay in Darwin and Modem 

 Science on the geographical distribution of plants, the result of which 

 is shown in the preceding chapter. 



Both Sphagnum and Carex respond to the same Law but in 

 different Degrees. — From the tabulated results for Sphagnum 

 and Carex given below, a few general inferences may be drawn. In 

 the first place, both groups of plants respond to the law involved 

 in the increase of dissimilarity between the east and the west as 

 one recedes from the Arctic regions, their differentiation intensifying 

 with the divergence of the land-masses from the north. Another 



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